It is known practice to dye keratin fibers, in particular human keratin fibers, by direct dyeing. The process conventionally used in direct dyeing comprises applying to the keratin fibers direct dyes which are colored or coloring molecules having an affinity for the fibers, allowing them to diffuse and then rinsing the fibers.
The direct dyes which are conventionally used are, for example, dyes of the nitrobenzene type, anthraquinone dyes, nitropyridine dyes, or dyes of the azo, xanthene, acridine, azine or triarylmethane type. These dyes can also be mixed so as to obtain varied colors. For example, it is known practice to use phenylpyrido[1,2-a]indolinium-derived dyes combined with red or blue direct dyes so as to obtain shiny dark or even black hair (EP 1 133 975).
The lightening of the color of dark keratin fibers to lighter shades, by optionally modifying the shade thereof, constitutes an important demand.
Conventionally, in order to obtain a lighter coloring, a chemical bleaching process is used. This process comprises treating the keratin fibers, such as the hair, with a strong oxidizing system, generally composed of hydrogen peroxide, possibly in combination with persalts, generally in an alkaline medium.
This bleaching system has the drawback of damaging the keratin fibers and of detrimentally affecting their cosmetic properties. The fibers in fact have a tendency to become rough, more difficult to disentangle and more brittle. Finally, the lightening or the bleaching of keratin fibers with oxidizing agents is incompatible with the treatments for modifying the shape of said fibers, particularly in hair straightening treatments.
Another lightening technique comprises applying fluorescent direct dyes to dark hair. This technique, described in particular in documents WO 03/028685 and WO 2004/091473, makes it possible to retain the quality of the keratin fiber during the treatment. However, these fluorescent direct dyes do not exhibit satisfactory fastness with respect to outside agents.
In order to increase the fastness of direct colorings, it is known practice to use disulfide dyes, in particular azo-imidazolium chromophore dyes in patent applications WO 2005/097051 or EP 1647580, and pyridinium/indolinium styryl chromophore dyes in patent applications WO 2006/134043 and WO 2006/136617. None of those documents mentions the problem of lightening keratin fibers without the use of chemical oxidation agents.